In modern gas turbines, requirements for high efficiency and output have resulted in significant increases in operating temperatures. This, in turn, has led to the design and construction of composite rotor structures using different materials. It has also led to the development of numerous complicated and costly internal flow circuits for delivering cooling air to the various portions of the gas turbine, including those exposed to the hot gas, to accommodate operation at increased temperatures.
A major problem in high-efficiency, high-temperature gas turbine operation has been non-uniform heating and cooling of rotor discs and the spacer discs which are sandwiched between adjacent rotor discs. For example, during transient operating conditions, i.e., start-up and other changes in speed between start-up and the turbine's rated speed, there is a significant temperature differential between the outer peripheral parts of the turbine discs, including the buckets, and the inner portions of the rotor discs. Similar temperature differentials occur between the outer and inner portions of the spacer discs. These essentially radially oriented, thermal gradients cause high thermal stresses which, in turn, can shorten the useful lives of the components. With specific regard to the spacer discs, damage to the conventional high/low sealing periphery of the spacer disc (which cooperates with an adjacent stationary structure with a complementary high/low surface configuration) cannot be readily repaired, leading to expensive rotor teardowns and even scrapping of the entire spacer disc.
This invention relates to a thermal sealing arrangement which substantially prevents hot interstage gases from impinging on the peripheral surfaces of the spacer discs in a gas turbine, thereby creating lower temperature gradients and stresses, and thus extending the service life of such discs and providing other benefits as explained below.
Specifically, the present invention provides for the removable attachment of high temperature sealing segments about the entire circumferential periphery of the rotor spacer discs. To this end, the conventional annular grooves provided in the peripheral surface of each spacer disc (as part of the high/low sealing surface configuration) are reconfigured as dovetail grooves which are designed to receive complimentary or mating portions of the sealing segments. In other words, each adjacent annular groove receives a plurality of sealing segments, the totality of which substantially prevent or at least minimize the impingement of hot gases directly on the peripheral surface of the spacer disc.
In another aspect of the invention, the utilization of discrete sealing segments on the periphery of the spacer discs also provides a structural basis for more advanced high/low sealing configurations.
At the same time, the potential for damage to the spacer discs per se is minimized in that the radial flexibility of the sealing segments permits the latter to radially retract upon the occurrence of any rubbing with the stationary component of the high/low sealing arrangement. Should wear occur on the segments themselves, they are easily removed and replaced, without time consuming teardowns.
Thus, in one exemplary embodiment of the invention, a plurality of dovetail type grooves are provided in the peripheral circumferential surface of each spacer disc of the gas turbine. Each groove slidably receives a plurality of sealing segments. Each segment has a base structure substantially complementary to the associated dovetail groove surfaces. Specifically, the base portion of each sealing segment includes a flat bottom surface, and a pair of inwardly and upwardly tapered surfaces which join with an upper portion of the segment which includes a substantially horizontal portion which is itself provided with an annular peripheral groove to maintain a high/low sealing surface configuration for establishing the otherwise known flow resistant path in cooperation with the complimentary surface configuration on the adjacent stationary structure.
The clearance between the surfaces of the lower or base portion of the sealing segments and the dovetail grooves is such that during rotation of the spacer disc, the sealing segments will be thrust radially outwardly as a result of the centrifugal forces exerted thereon, so that the upwardly and inwardly inclined surfaces of the sealing segments will sealingly engage the corresponding tapered surfaces of the dovetail grooves. This sealing engagement prevents direct impingement of hot interstage gases on substantial portions of the peripheral surface of the spacer disc. In this first described embodiment, it is possible for minimal amounts of hot gases to eventually contact other portions of the peripheral surface of the spacer disc by reason of clearance between adjacent segments in adjacent grooves. Nevertheless, the sealing segments provide a substantial heat barrier to the gases flowing in an axial direction across the spacer disc.
In another exemplary embodiment, the upper portions of the sealing discs are provided with asymmetrical horizontally extending flanges which are designed to sealingly engage similar flanges on adjacent segments in adjacent grooves so that a substantially complete thermal barrier is provided, preventing direct impingement of any hot interstage gases on the peripheral surfaces of the spacer disc.
In still another arrangement, a more advanced high/low sealing surface configuration is provided by profiling the upper portion of the sealing segments in a slightly more complex and asymmetric arrangement which presents a significantly more tortuous path for gases flowing axially across the surface of the spacer disc.
As a result of providing the thermal sealing arrangement as described above, the following advantages are obtained: (1) affective heat transfer resistance is provided to keep the rotor spacer discs at lower temperatures during transient conditions, thus reducing thermal stresses for improved service life; (2) more advanced high/low labyrinth seals are possible by utilization of segments profiled for optimum sealing effectiveness; (3) damaged or worn seals can be replaced in the field to reduce down time; (4) advanced seal designs can be incorporated into older field units for performance improvements; and (5) lower spacer disc thermal stresses permit a greater degree load sharing with adjacent turbine discs.
Other objects of the subject invention will become apparent from the detailed description of the exemplary embodiments of the invention which follows.